U.S. patent number 4,282,939 [Application Number 06/050,351] was granted by the patent office on 1981-08-11 for method and apparatus for compensating well control instrumentation for the effects of vessel heave.
This patent grant is currently assigned to Exxon Production Research Company. Invention is credited to James D. Howell, L. Donald Maus, Jerry M. Speers.
United States Patent |
4,282,939 |
Maus , et al. |
August 11, 1981 |
Method and apparatus for compensating well control instrumentation
for the effects of vessel heave
Abstract
A method and apparatus is disclosed for determining the flow
rate of drilling fluid flowing from a subaqueous well. The
invention may also be used to determine the presence of an abnormal
drilling condition during heaving motion of an offshore vessel from
which the drilling operation is conducted. The invention measures
the volume of fluid passing predetermined locations in the drilling
system at particular points in time so as to eliminate the
influence that the expansion and contraction of a telescoping
section has on the total volume of drilling fluid within the
system.
Inventors: |
Maus; L. Donald (Houston,
TX), Speers; Jerry M. (Houston, TX), Howell; James D.
(Gretna, LA) |
Assignee: |
Exxon Production Research
Company (Houston, TX)
|
Family
ID: |
21964759 |
Appl.
No.: |
06/050,351 |
Filed: |
June 20, 1979 |
Current U.S.
Class: |
175/7; 175/48;
73/152.21 |
Current CPC
Class: |
E21B
21/08 (20130101); E21B 21/001 (20130101) |
Current International
Class: |
E21B
21/00 (20060101); E21B 21/08 (20060101); E21B
007/12 () |
Field of
Search: |
;175/5,7,27,38,48,50,321,72 ;73/153,155 ;166/336,352,355,367 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Leppink; James A.
Assistant Examiner: Favreau; Richard E.
Attorney, Agent or Firm: Delflache; Marc L. Hammar; John
C.
Claims
What is claimed is:
1. A method for determining the flow of a drilling fluid during a
floating drilling operation having a marine riser and a telescoping
section, said method comprising:
determining a reference position on the telescoping section;
and
measuring the volume of drilling fluid flowing from said
telescoping section during the time period in which said
telescoping section moves away from and back to the reference
position.
2. The method according to claim 1 wherein said method includes
determination of the flow rate of drilling fluid flowing from said
telescoping section into said riser, said method further
comprising:
measuring the length of time during said time period; and
determining the flow rate of drilling fluid flowing from said
telescoping section into said riser by dividing said measured
volume of drilling fluid by said length of time of said time
period.
3. A method for detecting an abnormal drilling condition in a
floating drilling operation having a marine riser, a telescoping
section, a drill string extending through said riser and
telescoping section, and means for pumping drilling fluid down
through said drill string and up through said riser and telescoping
section, said method comprising:
(a) determining a reference position on the telescoping
section;
(b) measuring the volume of drilling fluid flowing from said
telescoping section during the time period in which said
telescoping section moves away from and back to the reference
position;
(c) measuring the volume of drilling fluid pumped into said drill
string during said time period; and
(d) comparing the volume measured in step (b) with the volume
measured in step (c), a difference between the two indicating an
abnormal drilling condition.
4. A method for determining the flow rate of drilling fluid flowing
from an offshore well into a marine riser wherein said well is
being drilled from a floating vessel wherein said marine riser
connects said vessel with said well, and a mud system in
communication with said riser by means of a conduit circulates said
drilling fluid within said well and riser wherein said marine riser
includes a telescoping section, having an upper and lower cylinder,
to accommodate vertical or heave movement of said vessel, said
method comprising the steps of:
sensing the relative position of said upper and lower cylinders
with respect to a predetermined relative position of said cylinders
wherein a first signal is generated when said cylinders vary from
said predetermined position initiating a clock, and a second signal
is subsequently generated when said cylinders return to said
predetermined position terminating said clock and defining a time
period between said first and second signals;
measuring the volume of said drilling fluid flowing downstream said
telescoping section during said time period; and
determining the flow rate of drilling fluid flowing from said well
into said riser by dividing said measured volume of drilling fluid
flowing downstream said telescoping section by said time
period.
5. The method according the claim 4 wherein said conduit is
maintained continuously full of drilling fluid, said method
comprising the further step of defining successive time periods by
generating successive signals and measuring a volume of drilling
fluid flowing downstream said telescoping section within each time
period thereby permitting the determination of multiple flow
rates.
6. A method for determining a kick or loss of circulation in an
offshore well being drilled from a floating vessel wherein a marine
riser connects said vessel with said well, and a mud system in
communication with said riser circulates a drilling fluid within
said riser, said riser includes a telescoping section, having an
upper and lower cylinder, to accommodate vertical or heave movement
of said vessel, said method comprising the steps of:
sensing the relative position of said upper and lower cylinders
with respect to a predetermined relative position of said cylinders
wherein a first signal is generated when said cylinders vary from
said predetermined position and a second signal is subsequently
generated when said cylinders return to said predetermined
position;
measuring the volume of drilling fluid entering said well from said
mud system between said first and second signals;
measuring the volume of drilling fluid flowing downstream said
telescoping section between said first and second signals; and
determining a kick or loss of circulation in said well when said
measured volume of drilling fluid entering said well from said mud
system is not substantially equal to said measured volume of
drilling fluid flowing downstream said telescoping section.
7. The method according to claim 6, said method comprising the
further steps of generating successive signals and measuring a
volume of drilling fluid entering said well from said mud system
and a volume of drilling fluid flowing downstream said telescoping
section between successive signals permitting the determination of
multiple differences between the measured volume of drilling fluid
entering said well from said mud system and the measured volume of
drilling fluid flowing downstream said telescoping section.
8. A method for determining the flow rate of drilling fluid flowing
from an offshore well being drilled from a floating vessel wherein
a marine riser, having a telescoping section with an upper and a
lower cylinder, connects said vessel with a well, and a mud system,
having a surge means, in communication with said riser circulates a
drilling fluid in said well and riser, said method comprising the
steps of:
sensing the relative position of said upper and lower cylinders
with respect to a predetermined relative position of said cylinders
wherein a first signal is generated when said cylinders vary from
said predetermined position initiating a clock, and a second signal
is subsequently generated when said cylinders return to said
predetermined position terminating said clock and defining a time
period between said first and second signals;
measuring the volume of drilling fluid in said surge means when the
clock is initiated;
measuring the volume of drilling fluid in said surge means when the
clock is stopped;
calculating the change in volume of drilling fluid in the surge
means;
measuring the volume of drilling fluid flowing downstream said
surge means during the time period; and
determining the flow rate of said drilling fluid flowing from said
well into said riser by dividing the sum of the measured volume of
drilling fluid flowing downstream said surge means and the measured
change in volume of drilling fluid in the surge means by the length
of time between the first and second signals.
9. The method according to claim 8 wherein said conduit is
maintained continuously full of drilling fluid, said method
comprising the further step of defining successive time periods by
generating successive signals and measuring a volume of drilling
fluid flowing downstream said surge means within each time period
permitting the determination of multiple flow rates.
10. A method for determining an abnormal drilling condition in an
offshore well being drilled by a floating vessel having a marine
riser which extends from said well to said vessel and a mud system,
having a surge means and in communication with said riser by means
of a conduit which extends from said mud system to said riser, said
mud system circulates a drilling fluid within said well and riser
wherein said conduit is continuously full of drilling fluid and
said riser includes a telescoping section, having an upper and
lower cylinder, to accommodate vertical or heave movement of said
vessel, said method comprises the steps of:
sensing the relative position of said upper and lower cylinders
with respect to a predetermined relative position of said cylinders
wherein a first signal is generated when said cylinders vary from
said predetermined position, and a second signal is subsequently
generated when said cylinders return to said predetermined
position;
measuring the volume of drilling fluid entering said well from said
mud system between said first and second signals;
measuring the volume of drilling fluid in said surge means when the
first signal is generated;
measuring the volume of drilling fluid in said surge means when the
second signal is generated;
calculating the change in volume of drilling fluid in the surge
means;
measuring the volume of drilling fluid flowing downstream said
surge means between said first and second signals; and
determining an abnormal drilling condition in a well when the sum
of said measured volume of drilling fluid flowing downstream from
said surge means and said measured change in the volume of drilling
fluid in said surge means is not substantially equal to said
measured volume of drilling fluid entering said well from said mud
system.
11. The method according to claim 10 wherein said method further
comprises the steps of generating successive signals and measuring
a volume of drilling fluid entering said well from said mud system
and a volume of drilling fluid flowing downstream said telescoping
section between successive signals permitting the determination of
multiple differences between the measured volume of drilling fluid
entering said well from said mud system and the measured volume of
drilling fluid flowing downstream said telescoping section.
12. In a system for offshore drilling from a floating vessel having
a marine riser which extends from the well to the vessel and a mud
system which is connected with said riser by means of a conduit
wherein said mud system circulates drilling fluid through said well
and riser, said riser includes a telescoping section having an
upper and lower cylinder to accommodate vertical or heave movement
of said vessel, the improved apparatus for determining a flow rate
of drilling fluid flowing into said riser from said well
comprises:
means for sensing the relative position of said upper and lower
cylinders with respect to a predetermined relative position capable
of emitting signals when said cylinders are momentarily in said
predetermined postion;
means measuring time between successive signals defining a series
of successive time periods; and
first means for measuring the volume of drilling fluid flowing
downstream said telescoping section during any of said time periods
permitting the determination of the flow rate of drilling fluid
flowing from said well when the volume measured within a particular
time period is divided by that time period.
13. The improved apparatus according to claim 12 wherein said
sensing means comprises a switch means to monitor said cylinders
and emit said signals.
14. The improved apparatus according to claim 13 wherein said
switch means comprises:
a potentiometer to monitor the relative movement of said cylinders
and generate a voltage varying as a function of said relative
movement;
a filter means connected to said potentiometer to eliminate all
voltage variations above a predetermined frequency permitting all
filtered voltage variations below said predetermined frequency to
pass;
means for determining the difference betwen said filtered voltage
variations and said generated voltage variation of said
potentiometer to define a modified zero axis about which said
voltage varies wherein said modified zero axis is representative of
said predetermined position; and
means for converting said varying voltage into discrete signals
when said voltage crosses said zero axis thus providing for a
signal when said cylinders are in said predetermined position.
15. The apparatus according to claim 12 further comprising:
second means for measuring the volume of drilling fluid entering
said well from said mud system during each of said time periods;
and
means for determining a difference between the measured volumes of
said first and second means during identical time periods wherein a
difference indicates the presence of an abnormal drilling
condition.
16. In a system for offshore drilling from a floating vessel having
a marine riser which extends from the well to the vessel, and a mud
system which connects with said riser by means of a conduit wherein
said mud system circulates drilling fluid through said well and
riser and wherein said riser includes a telescoping section having
an upper and lower cylinder to accommodate vertical or heave
movement of said vessel and said mud system includes a surge means
to attenuate a sudden change in volumetric displacement of drilling
fluid within said telescoping section, the improved apparatus for
determining a kick or loss of circulation from said well
comprises:
means for sensing the relative position of said upper and lower
cylinders with respect to a predetermined relative position capable
of emitting signals when said cylinders are momentarily in said
predetermined position;
means for measuring the time between successive signals defining a
series of successive time periods;
first means for measuring the volume of drilling fluid flowing
downstream said surge means during any of said successive time
periods;
second means for measuring the volume of drilling fluid entering
said well from said mud system during any of said successive time
periods; and
third means for measuring a change in the volume of the drilling
fluid within said surge means during each of said succesive time
periods permitting the determination of the presence of either a
kick or loss of circulation of well when the summation of the
volume measured by said first means and the measured change in
volume measured by said third means during identical time periods
is not substantially equal to the volume measured by said second
means during the same time period.
17. In a system for offshore drilling from a floating vessel having
a marine riser which extends from the well to the vessel and a mud
system which is connected with said riser by means of a conduit
wherein said mud system is capable of circulating drilling fluid
through said well and riser, and wherein said riser includes a
telescoping section having an upper and lower cylinder to
accommodate vertical or heave movement of said vessel and said mud
system includes a tank means which contains drilling fluid in
communication with said conduit, during a non-circulating drilling
fluid mode, the improved apparatus for determining a kick or
drilling fluid loss comprising:
means for sensing the relative position of said upper and lower
cylinder with respect to a predetermined reference position wherein
said sensing means emits a signal when said cylinders are
momentarily in said reference position defining a series of
succesive time periods between successively generated signals;
means for measuring the volume of drilling fluid within said tank
means each time a signal is emitted from said sensing means;
and
means for correlating said measured volume in said tank means such
that a sudden change in the successive correlated volumes measured
over said series of successive time periods is indicative of a kick
or drilling fluid loss.
18. A method as defined in claim 8 wherein the volume of drilling
fluid in the surge means is measured by detecting the level of
drilling fluid in the surge means at the time of interest.
19. A method as defined in claim 10 wherein the volume of drilling
fluid in the surge means is measured by detecting the level of
drilling fluid in the surge means at the time of interest.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method and apparatus for determining
the flow of drilling fluid from a subaqueous well and, more
particularly, relates to a method and apparatus for determining an
abnormal drilling condition such as the initiation of a blowout or
the occurrence of lost circulation during heaving or vertical
movement of a floating vessel from which the drilling operation is
being conducted.
2. Description of the Prior Art
In drilling a well, particularly an oil or gas well, using rotary
drilling methods, a hollow drill string extends from the surface to
the bottom of the well. A drill bit is attached to the lower end of
the drill string. Drilling fluid (mud) is circulated from the
surface, through the drill string and orifices in the bit to an
annulus defined between the drill string and the inner surface of
the well. The mud then circulates upward through the annulus to the
surface where it enters one or more tanks for processing (e.g.,
drill cuttings removed, chemicals added) prior to recirculation
into the well.
In a drilling operation, the mud has several functions, the most
important being to restrain high pressure fluids within various
earth formations. Occasionally, the high pressure fluid intrudes
into the well and displaces the mud. This initial intrusion is
referred to as a kick. If this occurs, it is important that the
pressure condition be balanced as soon as possible; otherwise, the
high pressure fluid might flow up the well. This condition is known
as a blowout. However, if during the drilling operation a weak
earth formation is encountered, the hydrostatic pressure of the mud
may fracture the rock and the mud may disperse freely into the
formation from the well. This is termed lost circulation.
A blowout is most effectively prevented when the kick or initial
intrusion of formation fluid is quickly detected and limited before
this fluid displaces a significant amount of mud from the well.
Similarly, lost circulation is most effectively limited when the
initiation of the loss can be quickly detected and counteracted
before a significant amount of the mud has flowed from the well
into the formation. Time is of the essence in detecting these
abnormal drilling conditions which may become dangerous
situations.
Two basic methods are commonly used in the drilling industry to
detect kicks or lost circulation. One method is based on a
determination of the flow of mud from the well. The second method
is based on a determination of the volume of mud displaced from or
to the well.
The first method is to determine the rate of flow of drilling mud
returning from the well and to compare this rate with either (i)
the rate of return mud flow at earlier times or (ii) the rate of
mud circulating into the well. The former approach is commonly used
and is useful since the rate of circulating mud into the well often
remains essentially constant for long periods of time. The latter
approach has the advantage of compensating automatically for normal
changes in mud circulating rate. An increase in the return flow
rate of mud from the well above an equivalent increase in the rate
of circulation into the well is an indication of a kick. Similarly,
an unexplained decrease in the return flow rate is an indication of
lost circulation.
The second basic method centers on the determination of the volume
of drilling mud contained in mud tanks at the surface which are in
fluid communication with the well. These tanks generally fall into
one of two categories--active tanks or trip tanks.
Active tanks are those through which mud is circulated for removal
of drilled solids and other treatment prior to recirculation. The
volume in the active tanks is responsive to differences between the
volume of mud pumped into the well and the volume returning from
the well. Although a number of normal processes may affect this
volume (removal of drilled solids, addition of water or other
materials), an unexplained increase in the volume is an indication
of a kick while an unexplained decrease is an indication of lost
circulation.
The trip tanks are usually much smaller than the active tanks and,
therefore, much more sensitive to changes in mud volume. They are
connected to the well during periods when no mud is being
circulated into the well through the drill string. Such
non-circulating periods include (i) times when a kick is suspected
and the mud circulation is stopped to conclusively determine if the
well is flowing and (ii) times when the drill string is being
removed from or returned to the well. This latter operation is
known as a trip, hence, the name "trip tank". During trips, the
change in volume of the mud in the trip tank is compared with the
displacement expected due to insertion or removal of a given length
of drill string into or from the well. In this manner, unexplained
increases or decreases in trip tank volume may be interpreted as
kicks or lost circulation, respectively.
Unfortunately, drilling offshore wells from a floating vessel
complicates the monitoring of the return mud rate or surface mud
volume. The drilling vessel is connected to the well by a marine
riser which serves as an extension of the well between the sea
floor and the vessel. The mud returns to the vessel from the well
through an annulus defined between the outer surface of the drill
string and the inner surface of the marine riser. In order to
accommodate the heaving or vertical motion of the vessel, the
marine riser usually includes a telescoping section or slip
joint.
At sea, the heaving motion of the vessel oscillates the telescoping
section thereby extending and contracting it. In this manner, the
lower section (below the telescoping section) of the marine riser
remains stationary with respect to the sea floor while the upper
section of the marine riser oscillates with the vessel. The
oscillating motion of the telescoping section increases and
decreases the volume of the annulus and, hence, the volume of the
mud in the annulus returning from the well. The resulting
variations in the annular volume of the telescoping section affect
the measurements of the flow from the well if one wishes to monitor
the flow above the telescoping section. Typically, this is the case
since it is currently impractical to measure the flow in the marine
riser below the telescoping section due to the difficulties
associated with a rotating drill string.
The maximum and minimum flow rate of the mud induced by the
extension and contraction of the marine riser may be several times
larger or smaller than the actual or true flow rate from the well.
For example, variations may occur in the measured return flow rate
of mud from 2000 gallons per minute (gpm) in the reverse direction
(when the telescoping section is expanding) to about 5000 gpm in
the normal direction (when the telescoping section is contracting)
compared to a true return flow rate of mud from the well of about
1500 gpm. In addition, the variations in the volume of mud in the
telescoping section induces variations in the volume of mud
contained in those tanks in fluid communication with the riser.
These variations complicate an accurate assessment of increases or
decreases in the tank volume. Therefore, the cyclic variations in
the volume of the marine riser caused by the movement of the vessel
complicates an accurate assessment of an abnormal drilling
condition. The rapid determination of a blowout or lost circulation
condition is very difficult without a means to correct for the
effects of the variation in the length of the telescoping section
if one wishes to monitor the return mud flow or volume above the
telescoping section.
Gorsuch, in his U.S. Pat. No. 3,602,322, discloses a system for
sensing a variation between the input and output flows of a well
above some defined tolerable range. Gorsuch's system is one of the
more elementary patents in the field for determining a blowout or
lost circulation. However, its application is limited to a
motionless system, e.g. onshore. The Gorsuch system cannot
effectively deal with variations in the return flow rate of
drilling fluid resulting from the heaving motion of the vessel.
The following U.S. Patents have recognized the problem of
accurately assessing the true flow rate of the returning drilling
fluid when monitoring the flow rate from above the telescoping
joint due to the heaving motion of the offshore vessel:
U.S. Pat. No. 3,760,891--Gadbois
U.S. Pat. No. 3,910,110--Jefferies et al
U.S. Pat. No. 3,976,148--Maus et al
The Gadbois system monitors the return flow rate of the mud at the
vessel and generates an electrical signal porportional to that
return rate. The signal is then monitored and accummulated over
preselected, overlapping time intervals and compared with threshold
values to determine the occurrence of a kick or lost circulation.
The Gadbois system requires the preselection of a time interval
over which the accummulation is performed. The time interval is
constant. The Gadbois system, however, does not provide for the
monitoring of a telescoping section over time periods such that the
effect on the final determination of the flow of mud from the well
due to the expansion and contraction of the telescoping section is
eliminated.
Jefferies et al (U.S. Pat. No. 3,910,110) discloses a system for
detecting a kick or lost circulation in a subaqueous well in which
the return rate of the mud flowing back to the vessel from the well
is measured and an electrical signal is generated proportional to
that rate of flow. The electrical signal is modified to compenste
for rates of change in the mud volume within the telescoping
section. The modified electrical signal is then compared with a
second electrical signal proportional to the rate of flow of the
mud into the well. U.S. Pat. No. 3,910,110 discloses a system for
continuously modifying the electrical signal which compensates for
a change in the volume of the flow path caused by the heaving
motion of the vessel.
Maus et al (U.S. Pat. No. 3,976,148) also discloses a method and
apparatus for determining on board a heaving vessel the flow rate
of drilling mud flowing from a well. However in the Maus
disclosure, a first, second and third electrical signal are
generated which correspond, respectively, to (i) a flow rate of mud
flowing through a conduit downstream the telescoping section, (ii)
a rate of change in the volume of mud contained within the riser
above the point at which the conduit between the mud processing
system and riser intersects the riser, and (iii) a rate of change
in the volume of the mud in the telescoping section. The first,
second and third signals are then correlated to produce a fourth
electrical signal proportional to the flow rate of the mud flowing
out of the well into the marine riser. U.S. Pat. No. 3,976,148,
however, requires the continuous monitoring of the extension and
contraction of the telescoping section to accurately assess the
rate of change in the volume of the drilling fluid passing through
the marine riser.
Other background references of a general interest relating to heave
compensation systems and pressure control of drilling fluid returns
are;
U.S. Pat. No. 3,809,170--Ilfrey et al.
U.S. Pat. No. 3,811,322--Swenson
U.S. Pat. No. 3,815,673--Bruce et al.
U.S. Pat. No. 3,946,559--Stevenson
U.S. Pat. No. 4,085,509--Bell et al.
U.S. Pat. No. 4,099,536--Dower
U.S. Pat. No. 4,099,582--Bell
U.S. Pat. No. 4,121,806--Iato et al.
U.S. Pat. No. 4,138,886--Lutz et al.
SUMMARY OF THE INVENTION
The present invention comprises a method and apparatus for
determining an abnormal drilling condition by determining the
intrusion of formation fluids into a well or the loss of drilling
fluid or mud from a well. Specifically, the present invention
comprises a method and apparatus for determining the flow of
drilling fluid flowing from a subaqueous well. The present
invention relates to offshore drilling from a floating vessel
having a marine riser which connects the vessel with the well. A
mud system is connected to the riser by means of a conduit. The
marine riser includes a telescoping or slip joint section having an
upper and lower cylinder relatively displaceable to accommodate
vertical or heave movement of the vessel. More specifically, the
present invention is concerned with determing the flow rate of
drilling fluid flowing from the telescoping section.
Briefly, the present invention comprises the step of measuring the
volume of drilling fluid flowing from the telescoping section
during the time period in which the telescoping section moves from
a predetermined reference position and returns to that
predetermined reference position. The reference position is a
preselected relative position of the upper cylinder with respect to
the lower cylinder. Thus, the volume of mud within the telescoping
section at the reference position is always constant due to the
preselected relative position of the two cylinders.
It is another feature of the present method to determine the
particular times at which the telescoping section is in the
reference position since the volume of mud contained within the
telescoping section at these times is the same. In this manner,
time periods or intervals are defined between those successive
points in time which indicate the occurrence of the predetermined
reference position. Thus, the flow of mud from the marine riser,
when averaged over one or more such time periods is unaffected by
vessel heave. The volume of mud contained in any tank in close
fluid communication with the marine riser, when measured at these
particular points in time, is also unaffected by vessel heave. In
other words, the volume in the telescoping section at successive
occurrences of the predeterminedreference position is unaffected by
vessel heave, eliminating the need to measure this volume and,
consequently, correct for a change in this volume (as required in
U.S. Pat. Nos. 3,918,110 and 3,976,148) in determining either the
flow of the mud leaving the well or the volume in the surface mud
tanks.
The present invention is applicable to various configurations of
piping between the marine riser and the mud processing system. In
one configuration the conduit connecting the marine riser with the
mud tanks is kept continuously full of drilling mud. In this
manner, errors which might result due to variations in mud volume
in the conduit between the marine riser and the point of flow or
volume measurement are minimized.
In another configuration, a surge means is used in fluid
communication with the conduit to attenuate the magnitude of
variations in the flow of drilling fluid caused by the extension
and contraction of the telescoping section during the heaving
motion. When employing a surge means, the measurement of the volume
of drilling fluid flowing downstream the telescoping section by
means of the present invention is preferably measured downstream
the surge means itself. The flow variations downstream the surge
means are substantially less than if a surge means were absent due
to the attenuating action of the surge means. Thus, when a surge
means is employed, the method of the present invention comprises
the step of measuring a change in the volume within the surge means
during the same time period that the flow downstream the surge
means is measured. The summation of the volume change in the surge
means and the volume passing the meter downstream the surge means
is the desired volume of mud flowing from the marine riser.
In another configuration, the surge means is utilized as a trip
tank. A valve is mounted within the conduit downstream the surge
means. When mud is not being circulated into the well, the valve is
closed thereby preventing flow through a meter downstream the surge
means. The volume of mud in the surge means, when measured at those
points in time indicative of the reference position is, therefore,
unaffected by the heaving motion of the vessel. Thus, unexplained
increases or decreases in this volume can be detected more rapidly
than heretofore possible.
In a preferred embodiment of the present invention, applicable to
piping configurations with or without a surge means, the time
period between successive reference positions is measured. The
measured volume of mud passing the flow meter (summed with the
change in volume of the mud within the surge means, if used) during
a particular time period is divided by the same time period to
determine the average rate of flow of mud from the marine
riser.
In another embodiment of the present invention, the volume of mud
contained within the active mud tanks is measured at the specific
times the telescoping section is in the reference position. The
piping configuration may or may not contain a surge means upstream
of the active mud tanks. If a surge means is used, the volume of
mud within it is also determined at the reference position and
summed with the volume in the active tanks.
It is a feature of the preferred embodiment, however, not to limit
itself to a single time period, but rather generate successive time
periods between successive signals. A signal is generated each time
the relative position of the upper and lower cylinders is at the
predetermined relative position. Each signal would terminate a
previous time period and initiate the subsequent one. Indeed, the
invention is not limited to equal time periods. The duration of
each time period is insignificant since the invention discretely
selects, by monitoring the telescoping section, time periods such
that change in the volumetric displacement of the telescoping
section due to heaving motion is eliminated. As used herein,
"successive" is distinguishable from "consecutive" in that the
invention is not limited to immediately adjacent time periods. The
invention may be practiced over multiple time periods defined to be
between a predetermined number of signals.
In practicing the invention, successive time periods may be
correlated with their respective volumetric measurements. A single
clock may record successive readings or alternatively, a series of
clocks each designed to initiate and terminate several meters, may
successively measure the change in volume of the surge means and
the volume of drilling fluid flowing downstream of the surge
means.
The improved apparatus for determining the flow of drilling fluid
flowing from a subaqueous well comprises, initially, a means for
sensing the relative position of the upper and lower cylinders with
respect to a predetermined relative position. The sensing means
emits a signal each time the cylinders are momentarily in the
predetermined position. The apparatus also includes a clock which
measures the time between successive signals defining a time period
or interval. In addition, the apparatus includes a means for
measuring the volume of drilling fluid flowing downstream the
telescoping section within said time period. In this manner, the
flow rate of drilling fluid flowing from the well is determined by
dividing the measured volume of drilling fluid flowing downstream
the telescoping section by the respective time period in which that
measurement was made.
In a modification of the apparatus, the measuring means includes
the ability to monitor, in series, a plurality of volumetric
measurements over successive time periods thereby enabling the
calculation of a series of flow rates flowing from the well by
dividing each volumetric measurement by its respective time
period.
In determining a kick or lost circulation, the apparatus may
include a means for measuring the volume of drilling fluid entering
the marine riser from the mud system during the time period. In
this manner, the occurrence of a kick or lost circulation is
indicated by a difference between the measured volume of drilling
fluid flowing downstream the telescoping section and the measured
volume of drilling fluid flowing into the well from the mud system.
In making this determination, however, both measurements of
volumetric flow must be made during the same time period.
In another modification of the apparatus in which a surge means is
included on the heave system, the improved apparatus would include
a means for measuring a change in the volume of the drilling fluid
within the surge means. The volume in the surge means would be
measured at those points in time wherein the telescoping section is
in the predetermined reference position. Successive readings would
be compared to assess a change in the volume. The volume of
drilling fluid flowing from the well is also measured downstream
the surge means. Therefore, in determining the presence of an
abnormal drilling condition, a difference between the measured
volume of the drilling fluid entering the riser from the mud system
and the summation of the measured volume of the drilling fluid
flowing downstream the surge means and the measured change in the
volume of the drilling fluid within the surge means, all measured
during the identical time period, is an indication of either a kick
or lost circulation. If a trip were made, the improved apparatus
would monitor merely the volume of the drilling fluid in the surge
means to determine if a change were occurring which might indicate
either a kick or loss of circulation.
BRIEF DESCRIPTION OF THE FIGURES AND TABLES
In order that the features of this invention may be better
understood, a detailed description of the invention as illustrated
in the attached figures and tables follows:
FIG. 1 is an elevation view of an offshore floating vessel drilling
a subaqueous well utilizing a conventional drilling fluid
circulating system.
FIG. 2 is a block diagram schematically illustrating the passing of
drilling fluid within the marine riser and undergoing volumetric
changes within a telescoping section of the marine riser due to the
heaving motion of the vessel.
FIG. 3 is an expanded elevation view of that portion of the
drilling fluid circulating system of FIG. 1 to which the present
invention applies.
FIG. 4 is another configuration of a drilling fluid circulating
system employing a rotating seal atop the marine riser and a surge
tank in communication with the conduit.
FIGS. 5A-5E illustrate sequential positions of the telescoping
section during a heave cycle.
FIG. 6 is a block diagram of the present invention used to
determine the flow rate and volume of drilling fluid flowing from a
subaqueous well in a manner which is unaffected by the heave of the
vessel.
FIG. 7 is a detail of a riser tensioner having a potentiometer
mounted thereto to sense the relative location of the upper and
lower cylinders of the telescoping section.
FIG. 8 is an electrical schematic of a switch means, employing the
potentiometer to monitor the relative location of the upper and
lower cylinders of the telescoping section and control the
processing of information by the invention.
FIG. 9 is yet another configuration of a drilling fluid circulating
system modified to employ the present invention. It is similar in
structure to FIG. 4 excepting the absence of the rotating seal.
FIG. 10 is a graph illustrating change in the length of the
telescoping section and several other parameters as a function of
time resulting from vessel heave.
FIG. 11 is a block diagram of the present invention used to
determine the presence of a kick or lost circulation in a
subaqueous well.
FIG. 12 is a block diagram of an alternate embodiment of the
present invention used to determine the presence of a kick or lost
circulation in a subaqueous well.
TABLE 1 is a tabulation of data assimilated from the Design
Example
TABLE 1 ______________________________________ Q.sub.ret * t** H**
.DELTA.H* .DELTA.V.sub.t * V.sub.m * V.sub.ret * .DELTA.t* (gal/
t** (sec) (ft) (ft) (gal) (gal) (gal) (sec) min)
______________________________________ t.sub.0 0 -.1691 t.sub.1
7.50 +.0671 +.2362 +12.52 62.48 75.0 7.50 600 t.sub.2 9.49 +.0303
-.0368 -1.95 21.85 19.9 1.99 600 t.sub.3 13.70 +.1551 +.1248 +6.61
35.49 42.1 4.21 600 t.sub.4 15.00 +.1232 -.0319 -1.69 14.69 13.0
1.30 600 t.sub.5 16.30 +.1050 -.0182 -0.96 13.96 13.0 1.30 600
t.sub.6 20.51 -.0655 -.1705 -9.04 51.14 42.1 4.21 600 t.sub.7 22.50
-.0215 +.0440 +2.33 17.57 19.9 1.99 600 t.sub.8 30.00 -.1691 -.1476
-7.83 82.83 75.0 7.50 600
______________________________________
TABLE 2 is a portion of the same data presented in TABLE 1 yet
computed over different time intervals.
TABLE 2 ______________________________________ Q.sub.ret * t** H**
.DELTA.H* .DELTA.V.sub.t * V.sub.m * V.sub.ret * .DELTA.t* (gal/
t** (sec) (ft) (ft) (gal) (gal) (gal) (sec) min)
______________________________________ t.sub.0 0 -.1691 t.sub.2
9.49 +.0303 +.1994 +10.57 84.33 94.9 9.49 600 t.sub.4 15.00 +.1232
+.0929 +4.92 50.18 55.1 5.51 600 t.sub.6 20.51 -.0655 -.1887 -10.00
65.10 55.1 5.51 600 t.sub.8 30.00 -.1691 -.1036 -5.49 100.39 94.9
9.49 600 ______________________________________
DETAILED DESCRIPTION OF THE INVENTION
Referring now to FIGS. 1-12, TABLES 1 and 2, and with particular
reference to FIG. 1, a vessel 20 is illustrated having a derrick 22
and derrick floor 29 mounted on the vessel 20 for subaqueous
drilling of a well 24 (also referred to as a well bore) offshore.
The vessel is connected to the well by means of a marine riser 26
which extends from the derrick floor 29 through a moon pool 28 in
the hull of the vessel 20 to the well bore at the sea floor. The
marine riser 26 is connected to the well 24 with typical blowout
preventive equipment (not shown) well known in the art. The marine
riser is connected at its upper end to the derrick floor 29 by
means of a riser-tensioning apparatus 30 which provides the upward
force necessary to support the riser.
The marine riser 26 also includes a telescoping section or slip
joint 32 near its upper end. This telescoping section comprises
upper and lower cylinders 34, 36 mounted for relative telescopic
movement such that the upper cylinder 34 moves within the lower
cyliner 36 during the vertical or heave movement of the vessel 20
due to the wave, tide and current influences. The tensioners 30
attach to the upper end of the lower cylinder 36. In this manner,
the vertical motion of the vessel is compensated for by the
tensioners 30. Tensioners are well known in the art. For example,
NL Shaffer's Riser Tensioner illustrated at page 4951 of Vol. III
of the Composite Catalogue of Oil Field Equipment and Services,
1978-1979 ed. published by World Oil. The upper cylinder 34 strokes
inside the lower cylinder 36 as the vessel oscillates. The lower
cylinder 36 remains stationary with respect to the sea floor.
A drill string 38 is supported from a swivel 40 which is, in turn
supported by a motion compensator 41 within the derrick 22. An
example of a motion compensator is NL Shaffer's Drill String
Compensator illustrated at page 4945 of Vol. III of the Composite
Catalogue of Oil Field Equipment and Services. The drill string 38
extends downwardly through the marine riser 26 into the well as
illustrated by the dashed lines. A drill bit 42 which is used to
drill the well is secured at the lower end of the drill string 38.
An annulus 44, defined by the inner surface of the marine riser 26
and the outer surface of the drill string 38, provides a return
flow path for the drilling mud. A conduit 50 intersects the upper
portion 25 of the riser 26 generally below the derrick floor 29,
and extends to a shale shaker 46 and active mud tanks 48.
A flow meter 45 is mounted on the conduit 50 between the riser 26
and the shale shaker 46. The flow meter 45 measures the rate or
volume of mud flowing toward the shale shaker 46. Measuring devices
(not shown) which are discussed in greater detail below, are
included to measure the volume of mud in the active tanks 48. A
standpipe 52 extends from the shale shaker 46 to a flexible hose 53
which in turn connects to the swivel 40 within the derrick. A flow
meter 56 is mounted on the standpipe to measure the rate or volume
of mud being injected into the drill string 38. A pump 54 takes
suction from the active tanks 48 and circulates mud up the
standpipe 52 through the flexible hose 53, to the swivel 40, down
the drill string 38 toward the bit 42 and back to the vessel
through the annulus 44. The returning mud exits the annulus thrugh
an aperture 27 in the riser 26 into the conduit 50 and flows to the
shale shaker 46 where the solids are screened out and the mud is
returned to the active tanks 48.
The mud exits the drill string 38 through the bit 42 flushing out
solids resulting from the drilling action of the bit while
simultaneously cooling the bit. The drilled solids are suspended in
the mud and are carried back to the vessel in the return flow up
the annulus 44. In order to maintain sufficient hydrostatic
pressure on the subterranean formations being drilled, the annulus
44 is maintained continuously full of mud.
As discussed previously, an abnormal drilling condition such as a
kick or lost circulation is detected by observing unexplained
changes in either the return rate of mud flowing from the marine
riser 26 or the volume of drilling mud in the active mud tanks 48.
In a stable state, e.g. on shore, the flow rate measured by the
meter 45 and the mud volume measured by instruments (not shown) in
tanks 48 are sufficient for indicating a kick or lost circulation.
An example of a mud volume measuring instrument, which is well
known in the art, is the Mud Volume Totalizer, Series MVTX,
manufactured by the Martin-Decker Company of Santa Ana, California.
In an ocean environment where having movement of the vessel 20
displaces the upper cylinder 34 of the telescoping section 32 with
respect to the lower cylinder 36, significant volumetric changes
occur which substantially affect these measurements and limit the
ability to detect potential well control problems.
By referring to FIG. 2, the phenomena which the marine riser is
actually undergoing is better appreciated. FIG. 2 diagramatically
indicates that, in any given period of time, the total volume of
incompressible fluid (V.sub.in) added to a saturated fluid system
is equal to the total volume removed from the system (V.sub.out)
plus any increase in the volume of the fluid within the system
(.DELTA.V.sub.sys). This relationship is mathematically represented
as:
Equation (1) can be applied to the entire mud circulating system as
shown in FIG. 1 or to any portion of it. For example, in analyzing
the effect of vessel heave, V.sub.in would be the volume of mud
entering the bottom of the telescoping section 32 and V.sub.out
would be the volume of mud flowing past the flow meter 45.
Therefore, .DELTA.V.sub.sys would be the change in the volume of
mud in the flow path between these two locations.
FIG. 3 is an enlarged drawing of a portion of the circulating
system of FIG. 1. As shown, the position of the aperture 27 of the
marine riser relative to the shale shaker 46 is such that portions
of the conduit 50, as well as the upper part 25 of the marine riser
26 may only be partially full of mud. A free surface 68 exists
within these components and this surface varies in elevation in
response to the flow surges caused by vessel heave. Therefore, the
term .DELTA.V.sub.sys for this system includes not only the volume
changes within the telescoping section 32, but also the changes in
mud volume within the upper part 25 of the marine riser 26 and the
conduit 50. Because of the generally uniform cross section of the
telescoping section 32, it is well known in the prior art to relate
the volume of mud within it to its overall length, which is easily
measured. However, the complex relationship between the free
surface 68 and the volume of mud contained within the upper part 25
of marine riser 26 and conduit 50 makes it impractical to measure
this volume.
The prior art, particularly Jefferies (U.S. Pat. No. 3,910,110) and
Maus (U.S. Pat. No. 3,976,148), teaches the modification of the
flow path between the telescoping section 32 and the flow meter 45
to either eliminate the free surface 68 (U.S. Pat. No. 3,910,110)
or create a geometry wherein the conduit 50 remains full of mud at
all times and the free surface 68 exists only in the upper part 25
of marine riser 26 above the aperture 27 (U.S. Pat. No.
3,976,148).
The prior art, particularly U.S. Pat. No. 3,976,148, teaches the
determination of the flow rate of drilling fluid entering the
bottom of the telescoping section according to the following
equation:
where Q.sub.in denotes the flow rate of drilling fluid entering the
bottom of the telescoping section, Q.sub.out represents the flow
rate of fluid passing through a flow meter in the conduit 50, and
dVsys/dt represents the continuous change in the volume of mud in
both the telescoping section 32 and upper marine riser 25 as a
function of time. In U.S. Pat. No. 3,976,148, both the length of
the telescoping section and the mud level above the intersection of
the conduit 50 and the riser are continuously monitored to directly
yield a change in the flow rate.
The present invention, which employs equation (1) in solving for
V.sub.in, depends on measuring V.sub.out during particular time
intervals chosen such that .DELTA.V is zero. In this manner, the
measured volume V.sub.out is an accurate indication of the volume
entering the telescoping section, V.sub.in, during the particular
time intervals. The average rate of flow of drilling fluid entering
the telescoping section may then be determined by dividing
V.sub.out by the length of the time interval or period over which
the volumetric measurement was made. Thus, the calculated average
flow rate past the meter 45 is substantially equal to the flow rate
entering the bottom of the telescoping section 32 and is unaffected
by vessel heave.
Alternatively, the volume of mud contained in tanks in fluid
communication with the marine riser can be measured at particular
times such that .DELTA.V.sub.sys is zero and the volume measured
is, thus, unaffected by heave.
FIG. 4 is a simplified drawing of an elevation view of a preferred
piping configuration with which the present invention may be
employed. The upper section of marine riser 26 includes the upper
and lower cylinders 34, 36 of the telescoping section. The conduit
50 intersects the marine riser 26 at aperture 27 and extends to a
shale shaker 46. The annulus area between the upper portion 25 of
marine riser 26 and the drill string 38 is sealed by means of a
rotating seal 58 such as the "Rotating Blowout Preventer"
manufactured by N. L. Shaffer of Houston, Texas and illustrated in
detail on page 4914 of Vol. III of the Composite Catalog of Oil
Field Equipment and Services. A surge tank 43 is in fluid
communication with the conduit 50 upstream of the shale shaker 46.
As shown in FIG. 4, the surge tank 43 is connected directly to the
conduit 50; however, the tank 43 may be connected directly to the
riser 26 via an outlet similar to aperture 27. A level sensor (not
shown, but for example, the Universal Trip Tank Monitoring System
Series TTSX, manufactured by the Martin-Decker Company of Santa
Ana, California) measures the height H of the free mud surface 68
in the surge tank 43. A flow meter 45 is mounted on the conduit 50,
preferably downstream of the surge tank 43. The configuration of
the conduit 50 in the vicinity of the flow meter 45 is chosen to
maximize the accuracy of the flow meter by maintaining the meter
full of fluid at all times and reducing the possibility of settled
solids blocking part of the meter flow area. The illustrated
configuration is predicated on the use of an electromagnetic-type
flow meter such as the Model 10D1435A/V Magnetic Flow Meter
manufactured by Fischer & Porter Company of Warminster,
Pennsylvania. Alternatively, a paddle-type flow meter such as the
"Flo-Sho" manufactured by Warren Automatic Tool Company of Houston,
Texas might be employed in a horizontal portion 47 of conduit 50
provided it is configured to operate pratically full of fluid.
Active mud tanks 48 receive the muds after passing through shale
shaker 46. These tanks also contain a level sensor such as
Martin-Decker's TTSX system. A valve 66 is located in conduit 50
downstream of surge tank 43.
The relative elevations of the seal 58, conduit 50, surge tank 43
and shale shaker 46 are chosen so that the mud surface 68 in the
surge tank 43 is the only free surface in the system. All other
components upstream of the flow meter 45 remain full of mud at all
times.
The surge tank 43 attenuates the magnitude of the surges
experienced by the flow meter 45, shale shaker 46 and active tanks
48. This is desirable since the unattentuated heave-induced surges
resulting from the expansion and contraction of the telescoping
section may be several times greater than the normal flow as
previously illustrated. By attenuating these surges, the components
need not be designed to handle these extreme magnitudes.
The present invention may be practiced on the configuration
illustrated in FIG. 4 to compensate for vessel heave when either a
flow measurement system or a volume measurement system is used. The
invention will first be described with respect to a flow
measurement system and then with respect to a volume measurement
system.
The application of Equation (1) to the system defined within the
dashed boundary outline 67 in FIG. 4 results in the following
equation:
where V.sub.ret is the return mud volume flowing into the bottom of
the telescoping section (corresponding to V.sub.in Equation (1)),
V.sub.m is the volume flowing out of the system past the flow meter
45 and V.sub. is the volume flowing out of the system into the
surge tank 43. The sum (V.sub.m +V.sub.t) corresponds to V.sub.out
in Equation (1). The term .DELTA.V.sub.ts is the change in the
volume of mud in the telescoping section and corresponds to
.DELTA.V.sub.sys in Equation (1).
In order to appreciate the significance of the present invention,
reference is made to FIGS. 5A through 5E which illustrate the
sequential operation of the telescoping section. The dashed line 33
represents typical heave-induced motion of the upper cylinder 34
which is attached to the vessel. For purposes of discussion, FIG.
5A is referred to as the reference position. The reference position
is chosen such that it is within the range of the stroking motion
of the telescoping section, preferably near the mid-point of the
cycle. At time T.sub.0, represented by FIG. 5A, the telescoping
section is in the reference position. The vessel is heaving
upwardly, as represented by the arrow 35, thereby extending the
telescoping section. FIG. 5B represents the relative orientation of
the cylinders at a later point in time wherein the vessel is near
the limit of its upward motion. Subsequently, the vessel heaves
downward and at time T.sub.1 the telescoping section is again in
the reference position, represented by FIG. 5C. The vessel
continues downward until, as represented by FIG. 5D, the motion
reverses and the vessel begins to move upward again. At time
t.sub.2 (FIG. 5E) the telescoping section is in the reference
position for the second time after t.sub.0.
A suitable switch or similar contact means (not shown) is connected
to the first and second cylinders such that each time the cylinders
are in the reference position (e.g. times t.sub.0, t.sub.1 and
t.sub.2 in FIGS. 5A, C, and E) the switch or contact means is
engaged emitting a signal. Thus, as the vessel heaves, signals are
generated indicating those times at which the telescoping section
is in the reference position. Typically, these signals will be
generated at intervals ranging from about one to eight seconds.
At those times the upper and lower cylinders are in the reference
position (hereafter occasionally referred to as "particular
times"), the .DELTA.V.sub.sys term in Equation (3) will be zero.
Therefore Equation (3) may be rewritten as:
wherein the asterisk (*) symbol designates quantities determined
over the intervals between the particular times (hereafter referred
to as "particular time intervals" or "time periods"). Thus, one
step in the method of the present invention is the determination of
the particular times at which the volume of mud in the telescoping
section is the same as in the reference position. By measuring the
volumes V*.sub.m and V*.sub.t during the particular time interval
between the particular times, the effect of the telescoping section
volume can be neglected. This differs significantly from the prior
art which requires that the volume of mud in the telescoping
section be measured.
Equation (4) indicates that the desired quantity, V*.sub.ret, which
is the volume of mud returning from the well, can be determined by
measuring V*.sub.m and V*.sub.t and summing these quantities. These
two volumes can be measured in numerous ways known to those skilled
in the art, two of which will be explained in more detail
later.
The rate of flow of drilling mud returning from the well is a very
useful indicator of kicks or lost circulation. This quantity can
also be conveniently computed by measuring the particular time
intervals, represented by .DELTA.t*. The average rate of return mud
flow into the bottom of the telescoping section over a particular
time interval, Q*.sub.ret, may be computed according to the
following equation:
Although Q*.sub.ret is an average, the method of this invention
differs substantially from other prior art methods which depend on
averaging over preselected relatively long periods of time to
reduce the observed magnitude of the heave-induced surges. These
prior art methods inherently result in a decrease in the
responsiveness of the flow measuring instrument to changes in the
flow from the well. In the present invention the averaging periods
are short and the compensation for heave complete; therefore, the
instrument is extremely sensitive to changes in the return flow,
with very little lag in its response.
The particular time intervals, .DELTA.t*, referred to above need
not be based on successive particular times. For example, referring
to FIGS. 5A-5E, .DELTA.t* may be the intervals t.sub.0 to t.sub.1
and t.sub.1 to t.sub.2, or the single interval t.sub.0 to t.sub.2.
In general, .DELTA.t* may be the interval between successive
particular times or some multiple thereof.
The foregoing has explained the principles involved in applying the
method of the present invention to compensate for vessel heave in
instrument systems in which the return flow of drilling mud is
determined to detect well control problems. The following
paragraphs describe the application of the present invention to
systems in which the volume of mud in tanks on the vessel is
determined to detect well control problems.
As previously explained, the mud volumes contained in tanks in
fluid communication with the well, e.g., the active mud tanks 48 of
FIG. 4, are responsive to kicks and lost circulation. For purposes
of this discussion, the volume in the tanks 48 (also known as
"pits") will be represented by V.sub.p. For an onshore well,
V.sub.p is the only volume in the mud circulating system which
varies significantly. However, in the configuration of FIG. 4, the
volume of mud in the telescoping section, V.sub.ts, the volume in
the surge tank, V.sub.t, and V.sub.p are all varying in response to
vessel heave. Further, the volume V.sub.t is responsive to kicks
and lost circulation in much the came manner as is V.sub.p. In
essence, the surge tank 43 can be considered an extension of the
system of active mud tanks.
In practicing the present invention with a volume measurement
system, it is necessary to measure the volumes V.sub.p and V.sub.t
at the particular times previously defined. The volume V.sub.ts
need not be measured since it is the same at each of the particular
times and is not responsive to well conditions. Referring to
Equations (1) and (3), this embodiment reduces to the
following:
where the double asterisk (**) symbol denotes measurements made at
the particular times and V** is the desired volume indication,
corrected for the effects of vessel heave.
A modification of the above method is possible when there is no
circulation of the well and a sensitive volume measurement is
desired. This can be used with either a flow or volume measurement
instrument system as noted above. In this instance, the in-line
valve 66 in FIG. 4 would be closed, isolating the flow meter 45 and
the active mud tanks 48 from the well. The surge tank 43 would then
to used as a trip tank. Since tanks 48 are isolated from the well
and cannot respond to kicks or lost circulation, Equation (6)
reduces to:
which indicates that the desired volume measurement is the volume
measurement of the surge (or trip) tank at the particular
times.
A preferred apparatus for the practice of the present invention is
shown diagrammatically in FIG. 6. The apparatus depicted is one
capable of providing both volume and flow outputs, compensated for
heave effects.
A heave switch 60 provides a signal at each particular time or
multiple thereof that the telescoping section cylinders are in the
reference position. A preferred apparatus for performing the
functions of the heave switch 60 is described in greater detail
later.
As indicated in FIG. 6, the output of the switch is a series of
pulses, corresponding to the particular times. Two of these pulses
occurring at particular times t.sub.1 and t.sub.2 are shown in the
graph 63 of FIG. 6. These pulses are command signals and are
transmitted via dashed lines 60a to an integrator 70, samplers 69
and 86, and counter 78.
The integrator 70 integrates the signal Q.sub.m from the flow meter
45 over the period between successive command signals (that is,
from t.sub.1 to t.sub.2) and transmits that integrated value at
t.sub.2. This output is the quantity V*.sub.m as previously
defined. A further function of the integrator is to begin a
subsequent integration at t.sub.2 and continue until the next
command signal. The output of integrator 70 preferably remains at
the value obtained during the interval t.sub.1 to t.sub.2 until a
subsequent integrated value is computed. In this manner, the output
of integrator 70 will be the value of V*.sub.m for the most
recently complete particular time interval.
The design of integrator 70, using either analog or digital
techniques, is well known to those skilled in the art. Its design
depends in part on the type of flow meter 45 used. A preferred flow
meter is the Model 10D1435A/U Magnetic Flow meter with two Model
50PZ1000A Flow Converters, manufactured by Fischer & Porter
Company of Warminster, Pennsylvania. These components may be
configured to produce a bidirectional flow meter, capable of
measuring flow in either direction. With this configuration, two
outputs are provided, one corresponding to the flow rate in one
direction and the other corresponding to the flow rate in the
reverse direction. These outputs each comprise a train of pulses,
the frequencies of which are proportional to the rate of flow of
mud through the magnetic flow meter in the respective direction.
Therefore, each pulse corresponds to a specific volume of mud. With
this type of flow meter, the integrator 70 is known as an "up-down
counter" which totalizes the pulses received during the particular
time interval. The counter will add those pulses (volumes)
corresponding to flow out of the system and subtract those pulses
(volumes) corresponding to reverse flow. In this manner, the
integrator 70 produces an output indicative of the net volume of
mud which has flowed out of the system past the flow meter,
V*.sub.m.
The sampler 69 samples an input value at the particular times as
indicated by the command signal and holds that value for output
until it is replaced by a subsequent input value. Analog and
digital samplers which perform this function are well known to
those skilled in the art. As indicated in FIG. 6, the input to the
sampler 69 is the volume of mud in the surge tank, V.sub.t. As
shown, this measurement is preferably obtained by measuring the
level H of mud in the surge tank 43 using a level sensor 62 and
scaling this measurement by the mud surface area, A.sub.t, in the
tank as indicated by an amplifier 64. A suitable device for
performing both of these functions is the Universal Trip Tank
Monitoring System, Series TTSX, manufactured by the Martin-Decker
Company of Santa Ana, California as mentioned earlier.
The output of the sampler 69, therefore, corresponds to V**.sub.t,
which is the volume in the surge tank 43 at the most recent
particular time. This quantity is utilized in computing both volume
and flow.
For flow determination, the output of the sampler is fed to the
input of a subtractor 72 which subtracts the most recent value of
V**.sub.t from that corresponding value measured at the immediately
preceding particular time. The output, therefore, is the value
V*.sub.t which represents the net increase in the volume of mud in
the surge tank 43 over the most recently completed particular time
interval. It can be seen by reference to FIG. 4 that this also is
the net volume of mud flowing out of the system defined by the
dashed line 67 into the surge tank 43. Of course, decreases in the
volume of the surge tank would be represented by negative values of
V*.sub.t.
As shown in FIG. 6, the output of the integrator 70 (V*.sub.m) and
the subtractor 72 (V*.sub.t) are added at a summing point 76. As
indicated by Equation (4), this sum is V*.sub.ret which is the
volume entering the bottom of the telescoping section over the most
recently completed particular time interval.
A third device receiving the command signal from the switch 60 is
the clock 78 which determines the period of time, .DELTA.t*,
corresponding to the most recently completed particular time
interval. These devices are well known to those skilled in the
art.
A divider 80 is connected to the summing point 76, the divider
receives the V*.sub.ret signal from the point 76 and divides it by
the corresponding particular time interval, .DELTA.t*. As
demonstrated by Equation (5), this computation results in the
desired flow rate Q*.sub.ret, unaffected by vessel heave.
For volume determination, the output V**.sub.t of the sampler 69 is
added to the sampled volume V**.sub.p from the active mud tanks.
This latter quantity is preferably determined by a pit volume
instrument 88 such as the Mud Volume Totalizer, Series MVTX,
manufactured by the Martin-Decker Company of Santa Ana, California
as mentioned earlier. The output, V.sub.p, of the pit volume
instrument 88 is fed to the second sampler 86. The operation of
this device is controlled by the command signal from the heave
switch 60 and is similar in structure and operation to the sampler
69 previously described. The V**.sub.p output is then added to the
V**.sub.t output at the summing point 82. The resulting value is
V** in accordance with Equation (6).
The apparatus of FIG. 6 contains two switches 74 and 84. Both are
operated in conjunction with the valve 66 in FIG. 4. They are shown
in their normal position, corresponding to the open position of the
valve 66. When the valve 66 is closed, these switches go to the
zero input position (represented by the ground symbol in FIG. 6).
This is the electrical analog of closing the valve and permits the
sensor 62 to monitor volumes and flows into the surge tank 43
alone. This mode corresponds to the "tripping" operation described
earlier.
It will be evident from the foregoing that heave switch 60 performs
a most critical function in the present invention. The switch means
may be a standard toggle switch mounted to the second cylinder
which is struck by a striker (not shown) mounted to the first
cylinder each time the cylinders are in the reference position.
Such a switch means is readily constructed by one skilled in the
art. Alternatively, the switch may be mounted to the riser
tensioners 30 (see FIG. 1) rather than the telescoping section
since relative movement of the tensioner with respect to the vessel
is similar in phase to the movement of the first cylinder with
respect to the second cylinder.
Rather than a toggle arrangement, the switch means may include a
cable-pull potentiometer. Such a potentiometer is well known in the
art and available from, for example, Humphrey, Inc. of San Diego,
California. As illustrated in FIG. 7, the tensioner 30 is attached
to the lower cylinder 36 by a cable 31. The lower cylinder 36 is
suspended from the vessel 20 by the tensioner 30 yet the cylinder
36 remains stationary due to the compensating function of the
tensioner 30. The tensioner 30 includes reciprocating cylinders
30a, 30b which coincide with the reciprocating movement of the
upper and lower cylinders 34, 36. The cable-pull potentiometer 33
measures a voltage variation stemming from the relative extension
of a cable 33a, attached to the cylinder 30a, with respect to the
potentiometer, attached to the cylinder 30b.
The electrical schematic of a switch means 60 is illustrated in
FIG. 8. The cable-pull potentiometer 33 generates an output voltage
which is an analog of the extension of the telescoping section.
While it may appear in the first instance that a signal
proportional to the length of the telescoping section is being
generated, it will become apparent that the analog voltage output
is used merely to eliminate the influence of tidal fluctuation and
generate command signals at the desired particular times.
Referring to FIG. 8, the output voltage from an wiper element 100
of the potentiometer 33 is split into two branches 102, 104. The
first branch 102 connects with an integrating circuit 106. The
circuit 106 serves as a low-pass filter. In other words, the
circuit 106 filters out all wave movement except very low frequency
movement, e.g. periods of over five minutes. This filtered signal
represents the reference position of the telescoping section. The
filtered signal from the circuit 106 is inverted and summed at
junction 108 with the original signal passing along the branch 104.
The summed voltage is then passed through an amplifier 110. In this
manner, the filtered signal from the circuit 106, when inverted and
summed with the potentiometer output voltage, reduces the voltage
to fluctuation about a modified zero voltage level (curve 111, FIG.
8) which negates the tidal influence by adjusting the zero level.
This eliminates having to physically relocate the switch on the
tensioner or the telescoping section. Thus, the circuit
automatically adjusts for such long-term variations as tidal
fluctuations. This is a principal advantage of the potentiometer
over the mechanical toggle switch as discussed above. Since
substantial tidal variation may relocate the natural zero line of
oscillation of the upper and lower cylinders 34, 36, contact
between the striker and contact might not occur without relocating
the mechanical switch.
The output signal from the amplifier 110 is fed into a Schmitt
trigger 112 whose output is a square-wave signal (curve 113) having
the same zero crossings and polarity as the input signal (curve
111). The square-wave signal is then fed into a differentiator
circuit 114 which converts the square-wave signal from the trigger
112 into a series of pulses (curve 115) occurring at each zero
crossing of the wave signal. The polarity of each pulse depends on
whether the zero crossing was positive-going or negative-going. The
output pulse from the differentiator 114 is fed into a
non-inverting buffer 116 and an inverting buffer 118. The output of
the non-inverting buffer is fed to a diode 120 which passes only
positive-polarity pulses. These pulses would correspond, for
example, to crossings of the reference position of the telescoping
section as it is extending (i.e., positive-going crossings). The
output of the inverting buffer 118 is fed to a similarly oriented
diode 122 which passes only positive pulses. However, because of
the voltage inversion, these pulses correspond to negative-going
crossings of the reference position (contraction of the telescoping
section).
The heave switch of FIG. 8 has two outputs, one corresponding to
positive-going crossings of the telescoping section reference
position and one corresponding to negative-going crossings. Either
one may be used for the command signal in FIG. 6. Alternatively,
these two outputs may be summed and the resultant used as the
command signal. In this event, a command signal would be generated
at each crossing of the reference position, regardless of the
direction of motion.
Referring back to FIGS. 5A-5E, a first signal or pulse is generated
at the positive-going output when the cylinders are momentarily in
the predetermined position as shown in FIG. 5A and the telescoping
section is extending. As the vessel rises due to the crest of an
ocean wave, the first cylinder advances upward with respect to the
second cylinder as illustrated in FIG. 5B. The vessel is then
lowered into the trough of the wave as it passes resulting in the
descent of the first cylinder with respect to the second cylinder.
At the time represented by FIG. 5C a signal is generated at the
negative-going output indicating that the telescoping section is
once again in the reference position and is contracting. Thus,
while a voltage from the potentiometer 33 is varying proportionate
to the length of the telescoping section, the output signals are
indicative of only the relative position of the cylinders with
respect to the reference position.
Based on the foregoing disclosure, the usefulness of the present
invention with the piping configuration represented by FIG. 4 is
apparent since the only assumption involved is that of
incompressibility of the drilling mud. Provided the mud does not
have significant quantities of entrained air or gas, this is a
reasonable assumption. There are, however, alternate piping
configurations with which the present invention may be used.
FIG. 9 is another configuration with which the present invention
may be used. It is similar to that of FIG. 4 except the rotating
seal 58 has been removed. Rotating seals may incur frictional
damage due to the heave-induced motion of the seal relative to the
drill string 38. Consequently, this component may require frequent
maintenance and its use may not be desirable. To prevent mud from
spilling out the open top end of the marine riser 26, the shale
shaker 46, surge tank 43 and conduit 50 have been lowered in FIG. 9
relative to their location in FIG. 4. The elevations of these
components relative to the outlet 27 of marine riser 26 are chosen
such that conduit 50 remains full of mud. Although the principles
involved in the design of these components are known, it is
recognized that constraints imposed by the overall design of the
drilling vessel may prevent achieving this objective under all
conditions of heave, mud flow and mud rheological properties. It is
anticipated, however, that the conduit 50 will remain full of mud
under most conditions.
The effect of the modifications illustrated in FIG. 9 creates an
additional free mud surface 59 in the upper part of the marine
riser 26. Obviously, if the level of this mud surface can be
measured, it could be treated as an additional surge tank according
to the principles disclosed above. However, under most conditions
this will not be necessary. The area of the free mud surface 59 in
the marine riser (hereafter designated (A.sub.r) is usually small
compared with the area A.sub.t of the free surface in the surge
tank. Consequently, errors in determining V*.sub.ret are not
anticipated to be large, even if free surface 59 were neglected.
However, recognizing that the level variations of the free surface
59 will be similar in magnitude an phase to those of the free
surface 68 in surge tank 43, a further improvement may be made by
considering the measurement of the surge tank level "H" (see FIG.
9) to be also a measurement of the mud level in the marine riser.
In the apparatus of FIG. 6, this could be accomplished simply by
increasing the gain of the amplifier 64 from A.sub. t to (A.sub.t
+A.sub.r).
Another piping configuration with which the present invention may
be employed as illustrated in FIG. 3. In this configuration, the
conduit 50 and the upper part of the marine riser 26 are generally
not full of mud, even under normal conditions. As stated above, the
complexity of this geometry makes it impractical to measure the
volume of mud within these components. However, by practicing this
invention, the largest component of error--the telescoping section
volume--is eliminated. For this application, the apparatus of FIG.
6 may be used by removing the level sensor 62, amplifier 64,
sampler 69 and subtractor 72, which are all associated with surge
tank volume measurements.
As noted above, it is well known in the art to detect well control
problems by comparing the rate of flow of drilling mud returning
from the well (Q*.sub.ret in the present invention) with either (i)
the rate of return mud flow at earlier times or (ii) the rate of
circulating mud into the well. The former comparison may be
accomplished in numerous ways with the present invention,
preferably by either recording the output Q*.sub.ret on a chart
recorder or by setting alarms which are activated whenever
Q*.sub.ret deviates by more than a predetermined amount from a
value determined during earlier trouble-free operations. The latter
comparison can be made in numerous ways well known to those skilled
in the art, two of which are illustrated below.
FIG. 11 depicts a modification of the apparatus of FIG. 6 in which
the rate of flow of mud circulating into the well, Q.sub.circ, is
substracted from the return mud flow rate Q*.sub.ret to yield a
delta flow parameter, .DELTA.Q*.sub.ret. Mathematically, this
corresponds to the equation:
The signal Q.sub.circ is obtained from the flow meter 56,
illustrated in FIG. 1.
An alternate approach for comparison of the flow rates is depicted
in FIG. 12. The output of the flow meter 56 is fed to an integrator
75 which functions similarly to the integrator 70. The integrator
75 integrates the signal Q.sub.circ over the period between
successive command signals (that is, from t.sub.1 to t.sub.2) and
transmits that integrated value as output at t.sub.2. The output is
the quantity V*.sub.circ which is the volume of mud pumped into the
drill string 38 during the particular time interval. This
integrated output is then subtracted from the sum of V*.sub.t and
V*.sub.m at summing point 76 to form the quantity
.DELTA.V*.sub.ret. Mathematically, this is expressed as:
where .DELTA.V*.sub.ret is the difference between the volume of
drilling mud which has entered the bottom of the telescoping
section over the particular interval and the volume of mud pumped
into the drill string 38 from the tanks 48 over the same interval.
A positive value of .DELTA.V*.sub.ret is indicative of a kick while
a negative value is indicative of lost returns.
The signal .DELTA.V*.sub.ret may be processed in several ways. For
example, it may be fed into the divider 80 as shown in FIG. 12 to
produce the delta flow parameter .DELTA.Q*.sub.ret, expressed
as:
Equation (10) is analagous to Equation (5). However, Equation (10)
concerns the determination of a difference, delta flow.
Alternatively, the .DELTA.V*.sub.ret values may be fed into a
totalizer 81 which adds each successive .DELTA.V*.sub.ret to the
total of previous values. Totalizers are well known in the art. In
this manner, the net volumetric gain or loss of mud from the well,
.SIGMA..DELTA.V*.sub.ret, is determined. This quantity is similar
to the change in the volume V** of mud in the surface mud system
but is unaffected by additions or deletions of materials from the
active mud tanks 48. In the preferred embodiment, the totalizer 81
would be activated whenever the parameter .DELTA.Q*.sub.ret
exceeded preset limits indicative of a well control problem. The
output .SIGMA..DELTA.V*.sub.ret would then indicate the total gain
or loss from the well from the inception of the problem. This
information is very useful in planning well control procedures.
DESIGN EXAMPLE
FIG. 10 is a graphical representation of data generated from the
embodiment of the invention disclosed in FIG. 6, and employed with
a configuration as illustrated in FIG. 4. The curves in FIG. 10 are
based on an upper cylinder 34 with an outer diameter of 18-5/8
inches, a drill string 38 with an outer diameter of 5 inches and a
surge tank 43 with a crosssectional area of approximately 7.08
square feet.
Curve I illustrates the change in length of the telescoping section
as a function of time. The adjusted zero axis represents the
reference position of the telescoping section.
Curve II is a squared curve illustrating the position of a
mechanical switch or the output 113 of the Schmitt trigger 112 in
FIG. 8 as a function of time. For example, starting at time t.sub.0
the telescoping section is in the reference position as indicated
by the location of Curve I at the zero axis. This position
corresponds with FIG. 5A. As the upper cylinder advances upwardly
the switch assumes Position 1 when the upper cylinder departs from
the reference position. The switch remains in Position 1 until the
telescoping section returns to the reference position at time
t.sub.1 at which point the switch position changes to Position 2.
The upper cylinder then continues downwardly, reaches a minimum
elevation, and then reverses its motion with respect to the lower
cylinder as illustrated in Curve I until time t.sub.2 when the
telescoping joint is again at the reference position. Therefore,
during all positive or upward displacements of the upper cylinder
with respect to the reference position, the switch is maintained in
Position 1. Similarly, all negative or downwardly displacements of
the upper cylinder with respect to the reference position maintains
the switch in Position 2. Obviously, Position 1 and Position 2 can
be represented by different voltage levels in an electronic
circuit.
Curve III is a pulse train obtained by differentiating Curve II.
Curve III illustrates the generation of a positive pulse 200 each
time the first cylinder is advancing upwardly and crosses the
reference position. A negative pulse 201 is generated each time the
first cylinder is advancing downwardly and crosses the reference
position. The use of these positive and negative pulses in
generating command signals is explained above with respect to FIG.
8.
A linear head-flow relationship was assumed in computing the
hydraulic behavior of this system as a function of time.
Consequently, Curve IV represents the behavior of two parameters.
As shown by the label of the left-hand ordinate, the graph
indicates the variation in the level H of the mud in the surge tank
about a mean level, arbitrarily assigned the value of zero. Because
this level (or head) supplies the driving force for flow through
the flow meter, Curve IV also represents the variation in this
flow, Q.sub.m. The right-hand ordinate indicates the magnitude of
these variations about a mean flow rate of 600 gal/min which is the
assumed flow into the bottom of the telescopting section. During
the expansion of the telescoping section between t.sub.0 and
t.sub.1 or t.sub.2 and t.sub.3, the volume of fluid in the surge
tank and the flow rate through the flow meter decrease, as
indicated by the values of H and Q .sub.m between t.sub.0 and
t.sub.1 or t.sub.2 and t.sub.3. Tank level and flow rate increase
during intervals of telescoping section contraction such as between
t.sub.1 and t.sub.2 and t.sub.5 and t.sub.6.
Referring to Table 1 for a more detailed tabulation of data from
the Design Example, the computed height at each of the particular
times (t.sub.0, t.sub.1, etc.) is indicated along with the computed
volume of fluid having passed the flow meter downstream the surge
tank during the preceeding particular time interval.
Knowing the mud level H** in the surge tank at each particular time
and the volume passing the flow meter V*.sub.m during each
particular interval, the flow rate of mud into the bottom of the
telescoping section can be determined as follows:
Between t.sub.0 and t.sub.1 :
Performing the above calculations for each particular time interval
yields a constant flow rate of 600 gallons per minute which is the
flow rate into the bottom of the telescoping section.
As noted above, the invention is not limited to the comparison of
immediately adjacent intervals. Rather, one may practice the
invention by skipping every other time interval of some multiple
thereof. Alternatively, one may practice the invention by combining
two or more intervals and redefining them as one interval. Or, one
may define a time interval between "like-sign zero crossings" by
using only the positive or negative pulses of Curve III. In other
words, at time t.sub.0 the slope of Curve I is positive since it is
increasing. At time t.sub.1 the slope of Curve I is negative since
it is decreasing. However, at time t.sub.2 the slope of the line is
once again increasing and, therefore, positive. A reading taken
between t.sub.0 and t.sub.2, termed "positive zero crossings", will
yield the flow rate when the respective mud elevations in the surge
tank along with the combined volume flow through the meter V*.sub.m
between t.sub.0 to t.sub.2 is considered. Performing such a
calculation yields a flow rate of, once again, 600 gallons per
minute.
Table 2 summarizes the specific values based on intervals between
"positive zero crossings" (positive pulses). The data from which
this table was generated are the same as for Table 1. It can be
seen that the volumes V*.sub.ret and the times .DELTA.t* are both
larger in Table II than in Table I. From the standpoint of
minimizing errors it is good practice to make the time intervals as
large as possible, consistent with maintaining adequate speed of
response in the instrument. It has been found that values of
.DELTA.t* of approximately 30 seconds represent a preferred time
frame. If the average heave period of the vessel were about eight
seconds, an interval consisting of eight consecutive reference
crossings (or four consecutive like-sign zero crossings) would be
desirable. In the Design Example, this interval would be from
t.sub.0 to t.sub.8.
The foregoing invention has been described in terms of various
embodiments. Obviously, many modifications and alterations based on
the above disclosure will be apparent to those skilled in the art.
It is, therefore, intended to cover all such equivalent
modifications and variations which fall within the scope of this
invention.
* * * * *